Download Population Genetics

Population Genetics
The modern theory of evolution recognizes
that the main source of variation in a population
lies in the differences in the genes carried by
the chromosomes.
Genes determine an organism’s appearance
and mutations can cause new variations to
arise.
These variations can be passed from one
generation to another.
Certain genotypes may be better equipped for survival
than others.
They may be better at obtaining food and water,
protecting themselves from predators or have a higher
reproductive potential.
When these organisms reproduce, these “successful”
genes will be transmitted to the offspring. The offspring
will be better able to survive; therefore, subsequent
generations would have an increased frequency of these
“successful” variant genes.
Consequently, there would be natural selection, within
the group, of individuals better adapted to prevailing
conditions.
Genes in Human Populations
• The principles of genetics, established by studies on
plants and fruit flies, can be applied to humans.
• However, the study of human genetics presents some
unique problems:
– Unlike garden peas and Drosophila, humans produce few
offspring, which makes it difficult to determine the genotypes of
both parents & offspring for any particular trait.
– Observing successive generations takes time. Drosophila can
reproduce every 14 days.
– Many human traits, including body size, weight and intelligence,
are affected by environment as well as genes.
 Population Sampling is one of the most
common ways to study human
populations.
A representative group of individuals
within the population is selected and
the trends or frequencies displayed
by the group are used as indicators
for the entire population.
Eg. Trait for tongue rolling is sampled this
way. Approximately 65% of the population
carry this dominant gene.
A population consists of all the members of a
species that occupy a particular area at the same
time.
Ex. Perch population of Lake Wabamun, dandelion
population of school field.
The members of a population are more likely to
breed with one another than with other populations
of the same species. Therefore, genes tend to stay
in the population for generation after generation.
The total of all the genes in all the members of a
population at one time is called the population’s
gene pool.
Evolution is the change in the frequency of genes
in a population’s gene pool from one generation to
the next.
Hardy-Weinberg Principle
• 1908 – G.H. Hardy & W. Weinberg,
independently derived the basic principle of
population genetics, the Hardy-Weinberg
principle.
• This law states that the frequencies of alleles in
a population’s gene pool remain constant over
generations if all other factors stay constant
(genetic equilibrium).
• Provides a model of an unchanging gene pool.
• Allele frequency is the proportion of gene
copies in a population of a given allele
• Changes in the population can be
measured by looking for changes in the
allele frequency
• A fixed frequency is when there is only a
single allele
The conditions under which no
change will occur are:
• 1) The populations must be closed
(no immigration/emigration)
• 2) Random mating takes place (no mating
preferences with respect to genotype)
• 3) No selection pressure. A single gene
must not affect the survival of the offspring
• 4) No mutations
• 5) Population must be large
If all of these conditions are met, the frequencies of two
alleles, for example A and a, will remain constant in a
population for an indefinite time (until conditions
change).
The mathematical expression for the Hardy-Weinberg
equilibrium is as follows:
p+q=1
p= frequency of the dominant allele
q= frequency of the recessive allele
p2 + 2pq + q2 = 1 genotype frequency
AA Aa aa
p2= frequency of genotype AA
2pq= frequency of genotype Aa
Q2= frequency of genotype aa
Example:
Suppose a certain allele A has a frequency of 0.6 in a population. The frequency of
allele a must be 0.4 because A + a must equal 1. (1 – 0.6 = 0.4). Let’s see what
happens during reproduction. We can arrange the alleles and their frequencies in
a Punnett square.
A (0.6)
a (0.4)
A (0.6)
AA (0.36)
Aa (0.24)
a (0.4)
Aa (0.24)
aa (0.16)
Genotype ratio: 36% AA; 48% Aa: 16% aa
Or use Hardy Weinberg:
Frequency of AA = p2 = (0.6)2 = 0.36
Frequency of Aa= 2pq = 2(0.6x0.4)= 0.48
Frequency aa = q2 =( 0.4)2 = 0.16
Unlike the genetic Punnett
square used to determine
individual traits, the eggs &
sperm of this Punnett
square represent the
genes for the entire
population.
Calculate the number of individuals of each genotype
in a population of 8000 individuals.
AA
Aa
aa
0.36
0.48
0.16
2880
3840
1280
Suppose a certain allele A has a frequency of
0.7 in a population (found in 70% of the genes).
Calculate the expected frequencies of the three
possible genotypes.
Therefore 0.7 = A frequency which is the
dominant allele = p
1- p(0.7) = 0.3
AA = 0.49 or 49%
p2 =(0.7)2 = 0.49
Aa = 0.42 or 42%
2pq = 2 (0.7 x 0.3) = 0.42
Aa = 0.09 or 9%
q2 = (0.3)2 = 0.09
If we were just given the distribution of genotypes, how
could we predict the frequency of the A and a alleles?
Ex.) Given the frequency of a recessive trait as 4%,
what are the allele frequencies?
q2 = 4% = 0.04
q = square root 0.04 = 0.2
p+q=1
1 - 0.2 = p (0.8)
p2 =(0.8)2 = 0.64
2pq = 2 (0.8 x 0.2) = 0.32
q2 = (0.2)2 = 0.04
Hardy-Weinberg purposely ignored the
external factors that influence populations.
The Hardy-Weinberg principle points out that
sexual reproduction reshuffles genes but does
not by itself cause evolution, which is caused
by a change in allele frequencies.
If the population does not demonstrate H-W
equilibrium, (i.e. it’s gene frequencies are not
stable), it is in evolutionary change!
Do Now
• What are the 5 conditions that must be
met so that genetic equilibrium stays the
same?
• Define:
– Population
– Gene pool
– Evolution
Factors that bring about
Evolutionary Change
• A population’s gene pool is very unstable. It
is constantly influenced by external factors –
factors that were intentionally ignored by Hardy
and Weinberg.
• These factors change a population’s genetic
makeup, upset the tendency toward genetic
stability & lead to evolutionary change.
• (microevolution – a change in the gene pool of a
population over successive generations).
1) Natural Selection
– The nonrandom survival & reproduction of certain
genotypes from one generation to the next.
– Certain traits may be selected for, while others may
be selected against.
– Ex. In North America, individuals who are
homozygous for normal hemoglobin have a selective
advantage over those who are heterozygous or
homozygous for the sickle cell allele.
– Eg. Peppered moth in England.
2) Mating Preferences
(mating may not be random)
• If females consistently choose to mate
with males having certain genetic traits,
they exert selection in favor of those traits.
3) Mutations
– A mutation is any inheritable change in the DNA of an
organism.
– Mutations occur in a cell as it undergoes meiosis to
form an egg or sperm.
– Two types:
• Chromosome Mutation – results from
nondisjunction, chromosomal breakage or
translocation
• Gene Mutation – changes in the nucleotides of a
DNA model
If the mutation gives selective advantage to
individuals carrying it, then it will increase in
frequency and the population gene pool will change
over successive generations.
Mutations, in and of themselves, are neither good nor
bad.
A mutation considered beneficial in one environment
may be detrimental in another environment.
Ex. Sickle Cell Anemia &
Tay-Sachs Disease
– Both homozygous recessive conditions.
– Carriers (heterozygotes) are usually symptom free.
– This is why disease still exists, or it would be selected out of
the population.
– Tay Sachs is a fatal genetic disease
– Sickle cell anemia can be treated (not cured), and
sometimes can be fatal
– African and south eastern Asian– high incidence of sickle
cell anemia and a high incidence of malaria. This is due to
the fact that sickle cell carriers provided immunity to the
malarial parasite.
– Eastern Jewish Populations – High incidence of Tay Sachs
and a high incidence of tuberculosis. Tay Sachs provided
immunity for the Jewish people from tuberculosis
4) Genetic Drift
– Evolution can occur simply by chance.
– Random events may bring death or lack of
parenthood to some individuals. As a result, alleles
may disappear from a population.
– Ex. Population of 10 guinea pigs. Only one member
displays an allele B, for black coat color. If the black
individual does not mate, the black allele will
disappear from the population.
– Genetic drift is more important in small populations
than in large ones. (H-W principle applies to large
populations and is based on the laws of probability
rather than natural selection).
– Founder effect is a genetic drift that results when a
small number of individuals separate from their
original population
5) Gene Flow (Migration)
– Movement of members into immigration, or
out of emigration, a population alters its
equilibrium.
– In immigration, new genes are added to the
population.
– In emigration, genes are removed from the
population.
Speciation
• Speciation refers to the formation of a new
species.
• There is an enormous diversification
between species that evolution alone
cannot explain.
• A group of similar organisms that can
interbreed and produce fertile offspring in
the natural environment.
It is important to note that speciation and
evolution are NOT necessarily the same.
Natural selection does not always cause
speciation! (Ex. The evolution of the peppered
moth did not lead to a new species).
How does speciation occur???
a. Instantaneous Speciation
• Occurring in one generation because of
major changes to the chromosomes
• Usually a result of nondisjunction
• Polyploids can mate with each other, but
not with members of parent generations,
because of different chromosome
numbers.
b. Gradual Speciation
• Most species arose slowly and gradually
evolved differences through time.
• i.e. Galapagos finches
c. Geographic Speciation
• Speciation occurs if a population is divided
into 2 or more smaller populations, that
are physically separated from one another.
• i.e. mountains or bodies of water from
floods establish physical barriers. Over
time the species cannot reproduce within
the original group
d. Punctuated Equilibrium
• Periodic rapid evolution (within 100-1000
generations ) followed by little change over
a long time
• Suggests that population remain stable
and unchanging for very long periods of
time
e. Phyletic Gradualism
• Evolution occurs at a constant rate over
time
Chapter 22
Population Changes
Populations and Communities
• a population refers to all of the individuals of
the same species living in the same place at a
certain time.
• a community is made up of the populations of
all organisms that occupy an area.
• the study of a community involves only the
organisms, whereas the study of an ecosystem
involves that abiotic and biotic components of an
area.
• A habitat is the physical area where a species
lives
• Within a habitat, every population occupies an
ecological niche – this is referred to as the
populations ecological role in the community,
including the biotic and abiotic factors under
which a species can successfully survive and
reproduce.
Do Now
• Make sure you have a calculator! If you
don’t have one, GET ONE NOW! 
• Also make sure you have your textbook
today.
Distribution of Populations
• Population patterns can be divided into three
patterns:
1. Clumped distribution
– occurs when individuals are grouped in patches or
aggregations
– organisms are distributed according to certain
environmental factors (abiotic factors)
– ex.) in river valleys, trees often grow only on the south
slopes and grasses dominate the north slope – plant
distribution is found in “clumps”
2. Random distribution
- occurs when there is neither attraction nor
repulsion among members of the
population
- arbitrary and not very common
3. Uniform distribution
- occurs when there is competition among
individuals for factors such as moisture,
nutrients, light and space
ex.) grass, cacti in deserts
Size and Density of Populations
• population size: the number of organisms of
the same species sharing the same habitat at a
certain time
• these numbers may arise from an exact count or
an estimate of the total population size using
sampling methods
• ex.) In 1981, there were 27642 northern pike in
Sylvan Lake, Alberta
• population density: the number of organisms
per unit space
• the density (D) of any population is calculated
by dividing the total numbers counted (N) by the
area (A) occupied by the population:
Dp = N
Dp = N
A
or
V (volume)
ex.) If 200 lemmings were living in a 25ha
(hectare) area of tundra near Churchill,
Manitoba in 1980, their population density
would be:
D=N
= 200 lemmings
D = 8 lemmings / hectare
A
25 hectares
• We can compare population densities by determining if
there have been changes within the same population
over a certain time period (we call this the rate of
change)
• Rate of density change can be expressed as follows:
Growth rate
= change in density or gr = ∆D
change in time
T
• D (gr) must be calculated showing the most recent
dates minus the density at the earliest date – this will
show whether there has been an increase or decrease
in the population. Same as growth rate formula!
ex.) If in 1990, the lemming population of
Churchill, Manitoba was 22 lemmings/ha and in
1980 the population density was 8 lemmings/ha
what is the change in population density from
1980 to 1990?
D 1980 = 8 lemmings/ha
D 1990 = 22 lemmings/ha
gr =  D = D 1990-D1980 = 22 lem./ha - 8 lem./ha
T
10 years
= +1.4 lemmings/ha/year
Note: must put + or – to show increase or
decrease!!!!
Population Growth Patterns
• Four factors determine population size:
1. Natality: the number of offspring of a species
born in one year
2. Mortality: the number of individuals of a
species that die in one year
3. Immigration: the number of individuals of a
species moving into an existing population
4. Emigration: the number of individuals of a
species moving out of an existing population
Population growth can be determined by the
following formula:
∆N= (natality (n) +immigration (i)) – (mortality (m)
+ emigration (e))
∆N =
(n+i)-(m+e)
x 100
initial # of organisms
∆N =
(b+i)-(d+e) x 100
n
Ex. If a colony of 200 cranes had 40
births & 55 deaths, with no migration,
what is the population growth?
• PG = 40 – 55 x 100
200
PG = - 7.5%
Population was deceasing.
Growth Rate
• How quickly a population is increasing or
decreasing
• Gr= ∆N
∆t
• Per Capita Growth Rate represents a
change in population size relative to the
initial size
• Cgr= ∆N
N
To Do
• Look over sample problem 1 on page 744
• Try practice questions 1 and 2 on page
745
• In mature ecosystems, populations tend to remain
relatively stable over the long term – this is called
dynamic equilibrium or steady state
• Dynamic equilibrium is similar to homeostasis;
populations will adjust to changes in the
environment to maintain equilibrium
• Populations can either be classified as “open” or
“closed”
• In open populations, all four factors (natality,
mortality, immigration and emigration) are
functioning
• In closed populations, immigration and
emigration do not occur, so changes in natality
and mortality will be the only factors that
influence population size
WE CAN RECORD
POPULATION CHANGES
USING GROWTH CURVES
J Curve Graph
• J-shaped population curves (ideal environment)
– If a few relatively active individuals are placed in an ideal
environment: unlimited space, food, water, without disease and
predation, the population can be expected to reproduce at its
maximum physiological rate.
– Only limiting factors would be the rate of gamete formation,
mating and survival of offspring.
– This would be shown on a J-shaped curve.
– results when rapid population growth is followed by a sharp
decline in the population
– the quick increase in population can cause an exceeding of the
carrying capacity, which then causes a sharp population
decrease
– it is usually followed by a relatively stable stationary phase
Doubling Time
• For any population growing exponentially,
the time needed for the population to
double in size is a constant
– td= 0.69
cgr
S Shaped Curve
• In real life situations, limiting factors curtail
growth and the curves tend to level off.
• Growth curves for open populations
– typically form “s-shaped” curves
A nutrient is added at the beginning of the
curve this results in a growth phase, followed
by a stationary phase
Once again where the curve levels off, the
maximum number of individuals that the
environment can support has been reached this number is now the new carrying capacity
(K)
Biotic potential (Rmax) is the maximum
number of offspring that can be produced by a
species under ideal conditions – this
determines the carrying capacity of that
There are six factors that regulate
biotic potential:
1.
Offspring (fecundity): the maximum number of
offspring per birth
2. Capacity for survival: the chances the organisms
offspring will reach reproductive age
3. Procreation: the number of times per year the
organism reproduces
4. Maturity: the age at which reproduction begins
5. Gender ratio – the more females, the greater the biotic
potential.
6. Mate availability
Factors that reduce population numbers (called
environmental resistance) limit population growth
Growth curves for closed
populations (limited resources)
–
•
–
•
–
–
–
–
•
–
four definite phases can be identified in this curve
LAG PHASE
the delay that occurs before the population enters a phase of
active reproduction
GROWTH PHASE
the population increases at its fastest rate during this phase
the rate of natality is greater than the rate of mortality
cell cultures and yeasts can grow exponentially (2, 4, 8,
16…)
the expected population increase in a given time (I) can be
calculated from the following formula:
I = growth rate (R) x current population (N)
eg.) If the growth rate of paramecia in a closed population
was 7.5% per day, and the initial population was 200, there
should be an increase of 15 paramecia on the first day (after
that it is compounded)
STATIONARY PHASE
the point where the population size no longer
increases
a lack of space, a shortage of nutrients and an
accumulation of toxic metabolic wastes cause a
reduction in the rate of increase
the rates of natality are equal to the rates of mortality
DEATH PHASE
the mortality rate exceeds the natality rate
nutrients run out and wastes accumulate
Density- Dependent Factors
• A factor that affects a population only
when it has a particular density
• This limits population growth
• The struggle for available resources within
a growing population would limit the
population size
Density- Independent Factors
• Populations may also experience changes
in size that are not related to population
density
• Changes in environmental conditions can
affect the size
• Examples: Temperature, Insecticides
Limiting Factors in Populations
• the law of the minimum states that “of the number of
essential substances required for growth, the one with
the minimum concentration is the controlling factor”
• Shelford’s Law of Tolerance states that “too little or too
much of an essential factor can be harmful to an
organism” – there is an optimal range of conditions for
maximum population size
• there are two general categories of limiting factors in an
environment:
– density independent: affect members of a population
regardless of population density (eg. flood, fire…)
– density dependant: factors that arise from population density
that affect members of that population (eg. food supply…)
r and K Population Strategies
• K selected populations:
– found in stable environments
– populations become crowded, causing intraspecific competition
– members are usually large in size and produce young that are
slow-growing and require parental care
– low reproduction rate
– eg.) elk, bear, humans
• r selected populations:
– undergo many unpredictable changes
– usually populations that are small in size
– have short life spans and a high reproductive rate
– the offspring grow rapidly and little parental care is needed
– a sudden environmental change can result is a large number of
deaths
• eg.) insects
Intraspecific and Interspecific
Competition
• “If two populations of organisms occupy the
same ecological niche, one of the populations
will be eliminated” – this is known as Gause’s
Principle and is due to interspecific competition
• interspecific competition occurs between
similar species for a limited resource (food,
water…)
• intraspecific competition occurs within an
ecological niche of members within the same
species
Predation
• two main ways for animals to avoid predation are:
• camouflage – an adaptation in form, shape or behavior
that better ables an organism to avoid a predator
• eg.) toxins produced by alder, birch or poplar trees deter
animals and insects
• eg.) butterflies taste bitter to birds
• eg.) During the winter, rabbits change their coloring to
blend with the environment
• mimicry – involves developing a similar color pattern,
shape or behavior that has provided another organism
with some survival advantage
• eg.) eyespots on butterflies
• coevolution can occurs between two species – this is
when the selection pressure
Symbiotic Relationships
• a symbiosis is a relationship in which two
different organisms live in a close
association
• there are three main types of symbiotic
relationships:
Symbiosis
• https://www.youtube.com/watch?v=zSmL2
F1t81Q
PARASITISM
– parasites obtain nourishment from their hosts,
but do not usually kill their hosts but often will
affect the host in a detrimental way
– eg.) Dutch elm disease – a parasitic fungus
uses the tree for food
Zombie Snails!
• https://www.youtube.com/watch?v=Go_LIz
7kTok
COMMENSALISM
– commensalism is an association between two
organisms in which one benefits and the other
is unaffected
– eg.) the fox and caribou in the arctic – fox will
often follow migrating caribou because the
caribou kick the snow out of the way so the
foxes will have a path to travel on
MUTUALISM
– a relationship in which two different organisms
live together and both benefit from a
relationship
– eg.) nitrogen fixing bacteria and legume
plants (biology 20!) – the plant feed the
bacteria sugar and the bacteria make nitrates
for the plant
– eg.) pollination
Life History Patterns
• Population cycles that include growth and decline
can occur in many populations
• The snowshoe hare and lynx have cycles that are
about 11 years in length
Population Histograms
• Population growth curves show how
populations change over time – not the
age distribution of the members
• With population histograms, we are able to
predict whether a population will grow,
stabilize, or decline
An age pyramid with a wide base is characteristic of a rapidly
growing population – it indicates a high number of young
offspring, but also shows the number of animals capable of
reproduction
population histograms with a narrower base are often approaching
zero population growth, and those with a more narrow base than
middle section are showing declining population growth
factors that affect population growth:
industrial revolution/technology
advances in medicine
weather
Chaos Theory
• Scientists are interested in studying very
complex phenomena which seem to defy long
term prediction. Eg. Biological communities &
populations, weather.
• A new way of examining why some features of
nature are so unpredictable is known as chaos
theory. This theory assumes that randomness is
a basic feature of many complex systems, longterm predictions may be extremely difficult.
• Even though features of nature are so unpredictable, they
often share similar characteristics:
• Outcomes of processes in a complex system are extremely
sensitive to small differences in the conditions that were
present when the process began.
• Once a process is underway, the relationships among the
interacting parts of the phenomena can change as a result of
the interactions themselves.
• Two systems that appear similar at the start may end up being
very different, but how the two will differ is unpredictable.
• Chaos is a normal feature in biological systems.
• The inability to predict the precise makeup of a community
does not mean that communities are entirely
unpredictable: communities tend to undergo predictable
changes over time called succession.
Succession
• Succession is the slow, orderly
progressive replacement of the community
by another during an areas development
• Succession ends by reaching a climax
community
• There are two possible types of
succession:
Primary Succession
– occurs in an area which no community
previously existed
– eg.) invasion of plant life of a newly formed
volcanic island
Secondary Succession
– occurs following the complete or partial destruction of
a community
– eg.) regrowth after a forest fire
– the first plants and animals to appear are called the
pioneer community
– lichen, mosses and insects are often considered
pioneer species
– pioneer communities develop into seral
communities which have plants and animals with
longer life cycles than pioneer species
– in the end, a climax community is formed where
there is a high rate of survival of all species
Some generalizations about
succession:
– species composition changes more rapidly during the
earlier stages of succession
– the total number of species increases dramatically
during the early stages of succession, begins to level
off during intermediary stages, and usually declines
and the climax community becomes established
– food webs become more complex and the
relationships more clearly defined as succession
proceeds
– both the total biomass and nonliving organic matter
increase during succession and begin to level off
during the establishment of the climax community